1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines.
2. Prior Art
Conventionally, an air-fuel ratio control system for an internal combustion engine is known, which is adapted to control the air-fuel ratio of an air-fuel mixture supplied to the engine in response to an output from an exhaust gas ingredient concentration sensor arranged in the exhaust system, the sensor having an output characteristic which is approximately proportional to the concentration of an ingredient (O.sub.2) in exhaust gases, to a desired air-fuel ratio set in response to operating conditions of the engine.
In an air-fuel ratio control system of this kind, the fuel injection period TOUT' (and hence the fuel injection amount) is controlled by correcting a basic value thereof by various correction coefficients such that the air-fuel ratio detected by the sensor (hereinafter referred to as "the supply air-fuel ratio") becomes equal to the desired air-fuel ratio. That is, in the above air-fuel ratio control system, the desired air-fuel ratio depends on varying operating conditions of the engine, so that correction coefficients are calculated based on engine coolant temperature TW, intake air temperature TA, and other engine operating parameters, respectively, and a basic fuel injection period TiM (read from a predetermined map) are multiplied by these correction coefficients by the use of the following equation (1') to calculate the fuel injection period TOUT': EQU TOUT'=TiM.times.(KTW.times.KTA.times.KWOT.times.. . . ).times.KLAF.times.KCMDM . . . (1')
where KTW represents an engine coolant temperature-dependent correction coefficient, KTA an intake air temperature-dependent correction coefficient, KWOT a high load correction coefficient, and KLAF an air-fuel ratio correction coefficient. Further, KCMDM represents a modified desired air-fuel ratio coefficient, which is generally obtained by multiplying a desired air-fuel ratio set according to the engine rotational speed NE and the intake pipe absolute pressure PBA by an air density-dependent correction coefficient KETC.
However, in the above air-fuel ratio control system, although the engine coolant temperature TW, the intake air temperature TA, etc. may largely change in response to the operating conditions of the engine, the fuel injection period TOUT' is calculated by multiplying the basic value TIM by numerous correction coefficients inlcuding those mentioned above, so that the fuel injection period TOUT' may unpreferably deviate from the optimum value. Particularly, in the case of so-called large-area feedback control in which the air-fuel ratio is feedback-controlled over a wide operating region or area of the engine by the use of a linear air-fuel ratio sensor (LAF sensor) as the linear output-type exhaust gas ingredient concentration sensor, it is additionally required to correct the fuel injection period even at the standing start of the vehicle (including idling), so that the number of multiplying terms, i.e. correction coefficients, increases, which makes it even more difficult to control the fuel injection period TOUT' to the optimum value in quick response to various operating conditions of the engine.
Further, the air-fuel ratio should desirably be accurately controlled in order to enhance the driveability, protect the engine, and reduce the fuel consumption. However, such accurate air-fuel ratio control is usually accompanied by complication of maps for obtaining suitable values of correction coefficients. For example, when the engine coolant temperature is low (e.g. during warming-up of the engine), the desired air-fuel ratio is generally required to be modified in the enriching direction to secure required driveability of the engine. To meet this requirement, it is necessary to provide a plurality of different maps for retrieval suitable for a high engine coolant temperature condition and a low engine coolant temperature condition, respectively, so that one of them may be selected according to the temperature conditions. This complicates the processing of calculation of the fuel injection period TOUT'.
Further, when the air-fuel ratio is to be shifted from a lean value to a rich value, it is necessary to once set the supply air-fuel ratio to a stoichiometric value and then shift it to a desired rich value, unless the engine is in a high-load condition, in order to avoid a drastic change in the air-fuel ratio, which may cause damage to the engine. This procedure for shifting the air-fuel ratio to an enriched value further complicates the processing of calculation of the related correction coefficient(s) (e.g. map retrieval).